However, when I choose the 400F, the airplane config window is still showing "PAX 3" at the main cargo door. I'm wondering if you guys are planning to fix that in the near future.
See your screenshot, GSX is telling you that taking the data from the User customization file under %APPDATA%. As explained on the manual, the User customization takes an higher priority over the GSX internal database.
So, if you created that configuration BEFORE the airplane was supported in the internal database, based maybe on a default airplane, it's likely to be wrong.
So, the first thing to do, is to press the RESET button, to restore the GSX configuration, so you can build on top of that. You cannot use the Pax 3 door, and make it "act" like a Cargo Door. What you must do, instead, is to CHANGE the coordinates of one of the two existing Cargo doors (best moving the AFT one), and set to the right, adjusting the height and be sure the green arrow is pointing "outside" the airplane.
If you don't want to find the name of XML variable for that door, you can just say "Door with no open/close controls", so GSX will still use it, but will ignore its open/door status.